Rapid advances in microelectronic devices are continuously demanding a finer pitch connection between electronic packages and a printed circuit board (about a few hundred micrometer pitch or less). To meet this demand as well as the demand for low cost electronic packages, surface mount technology (SMT) has expanded its share over the conventional plated-through-hole (PTH) technology for the last twenty years. At present, more than two thirds of integrated circuits (IC) including both memory and logic devices are assembled by SMT. SMT packages commonly found in a PCB are leaded chip carriers such as small outline integrated circuits (SOIC), plastic leaded chip carrier (PLCC), quad flat pack (QFP), thin small outline package (TSOP), or tape carrier package (TCP). These leaded chip carriers, mostly plastic packages, depend on a perimeter connection between an IC package and a PCB. The perimeter connection scheme of SMT packages has reached its limitation in terms of connection pitch and I/O capability.
To relieve the limitations of perimeter connections and thereby to increase the packaging density, area array connection schemes have recently become popular. Some of the area array packages developed for SMT include ball grid array (BGA) package, solder column grid array (SCGA), direct chip attach (DCA) to PCB by flip chip connection, tape ball grid array (TBGA), or chip scale packages (CSP). Among them, BGA is the most popular one, where solder balls connect a module carrying an IC to a PCB. This technology is an extension of the controlled collapse chip connection (C4) scheme originally developed for solder bump connection of multiple chips to a ceramic substrate.
The IC on the module can be connected to the module in several ways as taught by Mulles et al., U.S. Pat. No. 5,241,133; Massingill, U.S. Pat. No. 5,420,460; and Marrs et al., U.S. Pat. No. 5,355,283 among others. Ceramic or organic module substrates can be employed depending on the performance, weight and other requirements. The common feature, however, is that the connection between the IC carrier and the next level PCB is accomplished by an array of solder balls which are attached to the module by a solder alloy with a lower melting temperature.
BGA packages have several advantages over the conventional leaded chip carriers: small and low profile package, large, standard pitch for a same I/O count, high assembly yield due to self-alignment, rugged package (no lead deformation), better electrical/thermal performance, and compatible with SMT assembly process. A few drawbacks of BGA packages are noted such as a difficulty of visual inspection of solder joints, cost issues of BGA modules, control of solder ball connection process, lack of field reliability data, and others.
There are several options depending on the choice of module materials, such as plastic BGA, ceramic BGA, and tape BGA. Ceramic BGA is more costly than plastic BGA, but it has a better proven reliability over the plastic BGA. However, one major weakness of ceramic BGA is a large mismatch of thermal coefficient of expansion (TCE) between a ceramic module and a polymeric PCB. This limits the maximum size of ceramic BGA module to be mounted on a PCB, which is about 32 mm.sup.2 with state-of-art technology. For a ball pitch of 50 mil, this BGA module can have about 625 I/O connections. Plastic BGA is better in terms of TCE mismatch because of a better materials compatibility between the module and the PCB substrate materials. Since most of plastic BGA's have a perimeter connection to a chip by wire bonding, the overall packaging density is much lower than that of a ceramic BGA which has an area array connection to a chip by flip chip or C4 technology.
FIG. 1 schematically illustrates the cross section of a ceramic BGA module 13 connected to an organic substrate 15 by use of a two dimensional array of solder balls 14. On the BGA module 13, an IC chip 11 is already attached to it by another array of smaller solder bumps 12.
Solder balls used are typically 90% Pb-10% Sn in composition for ceramic BGA, 63% Sn-37% Pb eutectic solder for plastic BGA. As schematically shown in FIG. 2, the solder ball 24 is connected by reflowing Sn--Pb eutectic solder paste 23 as commonly used in SMT soldering. During the reflow of ceramic BGA, only the Sn--Pb eutectic solder paste 23 is melted, not the solder ball 24 of a high melting temperature. During the reflow process, several reactions occur simultaneously at the soldering interfaces; dissolution of terminal metalluriges such as Au or Cu layer into the molten Sn--Pb eutec tic solder, formation of Sn-containing intermetallic phases, interdiffusion of Sn and Pb across the liquid-solid interface, void formation in the solidifying Sn--Pb eutectic solder paste materials, and others. These reactions would affect the joint integrity or degrade the long-term reliability. The large mismatch in thermal expansion coefficients between a ceramic module 21 and an organic printed circuit board 26, often causes thermal fatigue failure along the interface 22,25 close to the ceramic module during a thermal cycling test. In order to improve the joint integrity and reliability, several solutions have been proposed; replacing Pb-rich solder balls with more flexible materials or placing a diffusion barrier layer on solder balls to prevent interdiffusion, or using a different joining material other than solders such as electrically conductive adhesives like Ag-filled epoxy.